In Search of Gravitomagnetism

April
20, 2004: NASA's Gravity Probe B spacecraft left Earth today in
search of a force of nature, long suspected but never proven: gravitomagnetism.

Gravitomagnetism
is produced by stars and planets when they spin. "It's similar
in form to the magnetic field produced by a spinning ball of charge,"
explains physicist Clifford Will of Washington University (St. Louis).
Replace charge with mass, and magnetism becomes gravitomagnetism.

We don't feel gravitomagnetism as we go about our everyday
lives on Earth, but according to Einstein's theory of General Relativity
it's real. When a planet (or a star or a black hole ... or anything
massive) spins it pulls space and time around with it, an action known
as "frame dragging." The fabric of spacetime twists like
a vortex. Einstein tells us that all gravitational forces correspond
to a bending of spacetime; the "twist" is gravitomagnetism.

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What does gravitomagnetism do? "It can make the orbits of satellites
precess," says Will, "and it would cause a gyroscope placed
in Earth orbit to wobble." Both effects are small and difficult
to measure.

Researchers led by physicist Ignazio Ciufolini have
tried to detect the gravitomagnetic precession of satellite orbits.
For their study, they used the Laser Geodynamic Satellites, LAGEOS
& LAGEOS II, two 60 cm diameter balls studded with mirrors. Precise
laser ranging of the pair allows their orbits to be monitored. The
researchers did find a small amount of precession consistent (at the
20% level) with gravitomagnetism. But there's a problem: Earth's equatorial
bulge pulls on the satellites, too, and causes a precession billions
of times greater than gravitomagnetism. Did Ciufolini et al.
subtract that huge pull with enough precision to detect gravitomagnetism?
Many scientists accept their results, notes Will, while others are
skeptical.

Gravity Probe B, developed by scientists at Stanford University,
NASA and Lockheed Martin, will do the experiment differently, using
gyroscopes.

The
spacecraft circles Earth in a polar orbit 400 miles high. Onboard
are four gyroscopes, each one a sphere, 1.5 inches in diameter, suspended
in vacuum and spinning ten thousand times per minute. If Einstein's
equations are correct and gravitomagnetism is real, the spinning gyroscopes
should wobble as they orbit the earth. Their spin axes will shift,
little by little, until a year from now they point 42 milli-arcseconds
away from where they started. Gravity Probe B can measure this angle
with a precision of 0.5 milli-arcseconds, or about 1%.

Any
angle measured in milli-arcseconds is tiny. Consider this: One arcsecond
equals 1/3600th of a degree. One milli-arcsecond is 1000
times smaller than that. The half milli-arcsecond precision expected
for Gravity Probe B corresponds to the thickness of a sheet of paper
held edge-on 100 miles away.

Sensing wobbles so small is a great challenge, and scientists working
on Gravity Probe B had to invent whole new technologies to make it
possible.

A
National Research Council panel, among them Cliff Will, wrote in 1995,
"In the course of its design work on Gravity Probe B, the team
has made brilliant and original contributions to basic physics and
technology. Its members were among the first to measure the London
moment of a spinning superconductor, the first to exploit the superconducting
bag method for excluding magnetic flux, and the first to use a 'porous
plug' for confining superfluid helium without pressure buildup. They
invented and proved the concept of a drag-free satellite, and most
recently some members of the group have pioneered differential use
of the Global Positioning System (GPS) to create a highly reliable
and precise aircraft landing system."

Not
bad for basic research.

Right:
GP-B's gyroscopes are the roundest objects ever made. Engineers at
the NASA Marshall Space Flight Center polished them to within 0.01
microns (less than 40 atom-widths) of perfect sphericity. Irregularities
must be eliminated; otherwise the gyroscopes could wobble on their
own without help from gravitomagnetism. [More]

Physicists are both anxious and excited by Gravity Probe B. They're
anxious because gravitomagnetism might not be there. Einstein's theory
could be wrong (a possibility held unlikely by most), and that would
spark a revolution in physics. They're excited for the same reason.
Everyone wants to be on hand for the next great advance in science.

Near Earth, gravitomagnetism is weak. That's why the Gravity Probe
B gyroscopes wobble only 42 milli-arcseconds. But gravitomagnetism
could be powerful in other parts of the Universe--for instance, "near
a spinning black hole or a neutron star," says Will. A typical
neutron star packs more mass than the Sun into a ball only 10 km wide,
and it spins a hundred thousand times faster than Earth. The gravitomagnetic
field there could be very strong.

Astronomers might have already observed the effects of gravitomagnetism.
Some black holes and neutron stars shoot bright jets of matter into
space at nearly light speed. These jets come in pairs, oppositely
directed, as if they emerge from the poles of a rotating object. Theorists
think the jets could be powered and collimated by gravitomagnetism.

Left:
An artist's impression of space and time twisting around a spinning
black hole. Credit: Joe Bergeron of Sky & Telescope magazine.

In addition, black holes are surrounded by disks of infalling matter
called "accretion disks," so hot they glow in the x-ray
region of the electromagnetic spectrum. There's mounting evidence,
gathered by X-ray telescopes such as NASA's Chandra X-ray Observatory,
that these disks wobble, much like the gyroscopes on Gravity Probe
B are expected to do. Gravitomagnetism again? Perhaps.

Here in our solar system gravitomagnetism is, at best, feeble. This
raises the question, what do we do with gravitomagnetism once we've
found it? The same question was posed, many times, in the 19th century
when Maxwell, Faraday and others were exploring electromagnetism.
What use could it be?

Today we're surrounded by the benefits of their research. Light bulbs.
Computers. Washing machines. The Internet. The list goes on and on.
What will gravitomagnetism be good for? Is it just "another milestone
on the path of our natural quest to understand nature?" wonders
Will. Or something unimaginably practical? Time will tell.

Editor's note: Physicist Clifford Will, whom
we interviewed for this story, is not a member of the Gravity Probe
B team. He is, however, an expert in General Relativity and an independent
reviewer of the Gravity Probe B project who has sat on several mission
review panels at the invitation of NASA.